16. Alternative Forms of Storage.
(i) Desiderate for storage systems. A storage system should have a high monetory economy, i.e. we wish to be able to store a large number of digits per pound sterling of outlay: it should also have a high spacial economy. For the majority of purposes we like a form of storage to be erasible, although there are a number of purposes, such as function tables and the greater part of the instruction tables, for which this is not necessary. For the majority of purposes we also like to have a short accessibility time, defining the accessibility time to be the average time which one has to wait in order to find out the value of a stored digit. Normally we shall be interested in the values of a group of digits which are all stored close together, and very often it does not take much longer to obtain the information about the whole group than about the single digit. Let us say that the additional time necessary per digit required is the digit time (reading). We may also define the accessibility and digit times for recording in the obvious analogous way, though they are usually either equal to the reading time or else exceedingly long.
(ii) Survey of available storage methods. The accompanying table gives very rough figures for the various available types of storage and the quantities defined above. This table must not be taken too seriously. Many of the figures are based on definite numerical data, but most are guesses. In spite of the roughness of the figures the table brings out a number of points quite clearly.
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cerebral cortex) are either very expensive and bulky, or else have a very high accessibility time.
(2) The really economical systems consist of layers packed into the form of a solid. They are read by exposing the layer wanted.
(3) The systems which are both economical and fairly fast have the information arranged in two dimensions. This apparently applies even to the cerebral cortex.
(4) Much the most hopeful scheme, for economy combined with speed, seems to be the ‘storage tube’ or ‘iconoscope’ (in J. v. Neumann's terminology).
(5) Some use could probably also be made of magnetic tape and of film for cases where the accessibility time is not very critical.
(iii) Storage tubes. In an iconoscope as used in television a picture of a scene is stored as a charge pattern on a mosaic, and is subsequently read by scanning the pattern with an electron beam. The electron beam brings the charge density back to a standard value and the charge lost by the mosaic registers itself through its capacity to a ‘signal plate’ behind the mosaic. The information stored in this way on an iconoscope, using a 500 line system, corresponds to a quarter of a million digits.
One might possibly use an actual iconoscope as a method of storage, but there are better arrangements. Instead of putting the charge pattern on to the ‘mosaic’ with light we can put it on with an electron beam. The density of the charge pattern left by the beam can be varied by modulating either the voltage of the signal plate or the current in the beam. The advantages of this are:
(a) The charge pattern can be set up more quickly with an electron beam than with light.
(b) Less apparatus is required.
(c) The same beam can be used for reading and recording, so that distortion of the pattern does not matter.
It seems probable that a suitable storage system can be developed without involving any new types of tube, using in fact an ordinary cathode ray tube with tin-foil over the screen to act as a signal plate. It will be necessary to furbish up the charge pattern from time to time, as it will tend to become dissipated. The pattern is said to last for days when there is no electron beam, but if we have a beam scanning one part of the target it will send out secondary electrons which will tend to destroy the remainder of the pattern. If we were always scanning the pattern in a regular manner as in television this would raise no serious problems. As it is we shall have to provide fairly elaborate switching arrangements to be applied when we wish to take off a particular piece of information. It will be necessary to stop the beam from scanning in the refurbishing cycle, switch to the point from which the information required is to be taken, do some scanning there, replace the information removed by the scanning, and return to refurbishing from the point left off. Arrangements must also be made to make sure that refurbishing does not get neglected for too long because of more pressing duties. None of this involves any fundamental difficulty, but no doubt it will take time to develop.